This invention relates to a novel image forming method with the use of a photosensitive member comprising an organic photoconductive material, particularly to an image forming method capable of forming images without fog by use of a toner containing positively chargeable nfine silica particles.
Various organic photoconductive polymers, including polyvinylcarbazole as a typical example, have been proposed as photoconductive materials to be used in electrophotographic photosensitive members. These polymers are superior to inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide in various respects such as film forming property, light weight, high productivity, etc. In recent years, in place of polymeric materials, a large number of organic materials of lower molecular weight have also been developed for organic photoconductive materials. Such a low molecular weight organic photoconductive material is advantageous in that it gives a photoconductive material having high sensitivity. This is realized by choice of a material having good sensitivity and charge retentivity from a broader scope of compounds which can be chosen. However, photosensitive members comprising organic photoconductive materials have the drawback of lesser surface hardness and being susceptible to flaws.
On the other hand, a positively chargeable toner to be used for development of negative latent images on a photosensitive member comprising an organic photoconductive material generally contains a positive charge controller. Such positive charge controllers include generally amino compounds, quaternary ammonium compounds, organic dyes, particularly basic dyes and salts thereof. Conventional positive charge controllers include benzylmethyl-hexadecylammonium chloride, decyltrimethylammonium chloride, nigrosine, safranine.gamma., and crystal violet. These are usually added into a thermoplastic resin, dispersed by melting under heating and micropulverized into fine particles, adjusted to suitable sizes, if desired, and then provided for use as a toner.
However, these dyes as charge controllers have complicated structures and do not have constant properties, thus being poor in stability. Also, decomposition or denaturation may occur through decomposition, mechanical shock and friction during kneading under heat or change in temperature and humidity conditions, to cause lowering in the charge controlling characteristic.
Accordingly, when development is carried out by use of a toner containing these dyes as charge controllers in a copying machine, the dyes may undergo decomposition or denaturation as the number of copies increases to cause deterioration of the toner.
As another serious disadvantage, it is very difficult to disperse these dyes as charge controllers evenly into a thermoplastic resin, and therefore difference in triboelectric charge is liable to occur among the toner particles obtained by crushing of the mixture.
Most of dyes for positive charge controlling are hydrophilic and therefore, due to poor dispersibility of these dyes into a resin, the dyes are exposed at the toner surfaces when pulverized after fusion kneading. When the toner is used under highly humid conditions, images of good quality cannot be obtained because of hydrophilic nature of the dye.
Thus, when a dye having positive charge controlling characteristic of the prior art is used in a toner, variances in amount of the charges generated on the toner particle surfaces through friction among between toner particles, between toners and carriers or between toners and toner carrying member such as sleeve, will occur, whereby various inconveniences are caused, such as development fog, toner scattering, carrier contamination, etc. Also, under highly humid conditions, the transfer efficiency of the toner image is markedly lowered to be unsuitable for practical use. Even under normal temperature and humidity, when the toner is stored for a long time, due to instability of the positive charge controlling dye employed, toner particles may frequently agglomerate to become useless.
The research group to which we belong has proposed a toner containing fine silica particles synthesized by a wet process suitable for development of negative latent images in Japanese Laid-open Patent Application No. 78549/1982 and others. When this toner was used, however, for development of negative images on a photosensitive member comprising an organic photoconductive material and a large number of copies were taken under an environment of low temperature and low humidity, it was found that fine silica particles were caused to adhere onto the surface of the photosensitive member and the copied images were susceptible to fogging. This may be attributable to the following reason. The fine silica particles synthesized by the wet process have greater primary particle sizes and further are more susceptible to agglomeration than fine silica particles synthesized by a dry process, thus forming larger particles (about 1 micron). Also, since the fine silica particles synthesized by the wet process are liable to be charged negatively, through friction with the toner particles, positive charges are given to the toner particles, while they are themselves charged negatively. As the result, the fine silica particles synthesized by the wet process tend to be attached onto the non-image portion on the latent image, and further, in the transfer step, cannot be transferred onto a transfer paper because of having the polarity opposite to that of the toner, but remain abundantly on the photosensitive member. Further, since the silica consists of fine particles and has also great hardness, the silica remaining on a photosensitive member is liable to form flaws on the photosensitive member. Therefore, when image formation is effected repeatedly, the silica particles will become attached onto the photosensitive member to cause fog on the copied image. This tendency becomes further pronounced under low humidity environment, because of increased triboelectric charge.
Further, for overcoming this problem, in various cleaning methods, such as the blade cleaning system, the fur brush cleaning system, the magnetic brush cleaning system, etc., measures have been investigated to give as little load as possible to the toner and the photosensitive member. However, it could not completely be overcome by such measures.